Amperometric sensor system

ABSTRACT

A sensor system that measures at least one parameter of water includes an electronics subsystem and includes a sensor housing electrically and mechanically coupled to the electronics subsystem. The sensor housing encloses a chamber that receives water via at least one inlet and that releases water via at least one outlet. At least one sensor has at least one electrode exposed to water in the chamber. A flow generator causes water to flow through the chamber. A plurality of objects within the chamber move in response to the water flow and abrasively clean the at least one electrode. Preferably, the sensor system includes a chlorine sensor having at least two electrodes. The electronics subsystem applies a first differential voltage between the two electrodes during a measurement interval and then applies a second differential voltage between the two electrodes during an interval following the measurement interval.

RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 13/372,260 filed on Feb. 13, 2012, which claims thebenefit of priority under 35 USC §119(e) to U.S. Provisional ApplicationNo. 61/443,240, filed on Feb. 15, 2011, to U.S. Provisional ApplicationNo. 61/548,953, filed on Oct. 19, 2011, to U.S. Provisional ApplicationNo. 61/597,762, filed on Feb. 11, 2012, and to U.S. ProvisionalApplication No. 61/597,832, filed on Feb. 12, 2012, which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of systems for testing waterchemistry, and, more particularly, is in the field of amperometricsensors.

2. Description of the Related Art

Amperometric chlorine sensors are used in measuring chlorine residualsin drinking water, wastewater, cooling towers. Some newer applicationsare measurement of Total Residual Oxidant (TRO) in seawater. Recentregulatory actions require treatment of ballast water onboard ships toinactivate invasive species and prevent their discharge into non-nativewaters. Another new application is the measurement of up 500 ppm TRO inseawater used for biofouling back flushes of pretreatmentmicrofiltration membranes used in Reverse Osmosis Desalination Systems.

A common problem encountered with online measurement of water chemistryin the field is fouled electrodes in the sensor system. Electrodemeasurements can be rendered unreliable when the working electrode iscovered with either inorganic (salts such as calcium carbonate) layersor organic (biofouling) layers that inhibit electrode processes.

Another problem with certain sensors is the lack of a flow independentmeasurement method that can installed in a process flow without adverseeffects from changing flow rates on the sensor signal. Amperometricsensors, in particular, are significantly affected by flow rates.Preferably, a sensor system should provide accurate measurements withflow rates ranging from 0 to more than 7 feet per second without anappreciable change in the sensor output signal. Another limitation ofmost sensors is the lack of an ability to directly insert a sensor intoa process flow pipe or fitting for a simple installation such as may benecessary in a drinking water distribution system.

Maintenance costs of online chlorine monitors often greatly exceed thecost of the unit. Frequent recalibration is necessary with most, if notall, commercially available sensors due to changing electrode surface,fouling, electrolyte depletion, membrane fouling or stretching, pressurechanges or spikes, flow changes and changes in pH.

Many sensor systems require a reagent feed of either an iodide solutionor a buffer to lower the pH to 4.0. Other systems are dependent on flowor pressure and require controlled flow with a drain to waste to providea constant flow rate to the sensor. This requirement further complicatesthe installation, maintenance and logistical requirements. This resultsin excessive water loss or unnecessary consumption. Solution consumptionand replenishment result in higher costs and maintenance requirements.

Polarization of many sensors causes a loss of sensitivity over the first2-24 hours, requiring recalibration. Sensitivity is reduced and thecalibration changes if the sensor is removed and replaced.

When amperometric systems with exposed electrodes are used with even lowlevels of cyanuric acid (CYA), a polarization of the electrode occurswith a layer of CYA that inhibits electron processes, rendering theelectrode unusable after less than one day of operation due to the lowsignal response.

Oxidation reduction potential (ORP) is often used for residual controland provides a qualitative indicator of sanitizer efficacy. The probesused for ORP sensing often suffer from a reduction in sensitivity causedby organics, particularly when high levels of cyanuric acid are present.An ORP sensor is needed that does not have this limitation.

The vast majority of commercial amperometric systems use membranes andelectrolyte to control the reaction that occurs at the working(measurement) electrode. Problems with these systems are well known.Membranes stretch and foul with oil and organics and must be replacedfrequently. Another problem with most of these systems is that theymeasure only hypochlorous acid, a free chlorine fraction, not freechlorine as measured by a DPD test kit. As a result, membrane sensors'signals change radically with a change in pH compared to bare electrodesensors. This results in a limited operational range of 6.0 to 8.0 pH.Beyond a pH of 8.0, the sensors produce very little signal, which canproduce large errors.

Electrolyte must be replaced on an ongoing basis, often monthly. Inaddition to these deficiencies, membrane sensors are affected by flowand pressure changes requiring recalibration when either changes.

A known sensor uses a replaceable thin-film sensor formed on a substratewith multiple electrodes and a screen printed membrane on the workingelectrode for chlorine. The sensor has a very short life ofapproximately 6 months.

Another issue with known sensors is the detection and/or interference ofchloramines in drinking water. Chloramine (monochloramine) is adisinfectant used to treat drinking water. Chloramine is most commonlyformed when ammonia is added to chlorine to treat drinking water. Thetypical purpose of chloramine is to provide a longer-lasting residualfor disinfection as the water moves through pipes to consumers. Thistype of disinfection is known as secondary disinfection. Chloramine hasbeen used by water utilities for almost 90 years, and the use ofchloramine is closely regulated. More than one in five Americans usesdrinking water treated with chloramine. Water that contains chloramineand that meets EPA regulatory standards is safe to use for drinking,cooking, bathing and other household uses.

Many utilities use chlorine as a secondary disinfectant; however, inrecent years, some utilities have changed the secondary disinfectant tomonochloramine to meet disinfection byproduct regulations.

Monochloramine (NH₂Cl) is commonly used in low concentrations as asecondary disinfectant in municipal water distribution systems as analternative to free chlorine chlorination. The use of monochloramine isincreasing. Chlorine (sometimes referred to as “free chlorine”) is beingdisplaced by monochloramine, which is much more stable and does notdissipate from the water before it reaches consumers. Monochloraminealso has a very much lower, however still present, tendency than freechlorine to convert organic materials into chlorocarbons such aschloroform. Such compounds have been identified as carcinogens; and in1979 the United States Environmental Protection Agency began regulatinglevels of such compounds in drinking water. Furthermore, water treatedwith chloramine lacks the distinct chlorine odor of the gaseoustreatment and so has improved taste. In swimming pools, chloramines areformed by the reaction of free chlorine with organic substances.Chloramines, compared to free chlorine, are both less effective as asanitizer and more irritating to the eyes of swimmers. When swimmerscomplain of eye irritation from “too much chlorine” in a pool, theproblem is typically a high level of chloramines. Some pool test kitsdesigned for use by homeowners are sensitive to both free chlorine andchloramines, which can be misleading.

The following chart illustrates the current versus the Cl potential andversus the monochloramine potential that is used to determine whichspecies is present:

Analytical Signal (μA) Potential (V) Chlorine Blank NH₂Cl Blank 0.301.179 0.016 0.034 0.008 0.20 1.400 0.016 0.009 0.004 0.10 1.6595 0.0270.248 0.028 0.00 2.287 0.241 0.745 0.311 Analytical Signal (μA) (corr.)Potential (V) Chlorine NH₂Cl 0.30 1.162 0.026 0.20 1.383 0.005 0.101.633 0.220 0.00 2.046 0.434

In the foregoing chart, the concentration of Cl is 20 parts per million(ppm), and the concentration of NH₂Cl is 20.5 ppm as Cl. The “blank”readings in the two columns are determined by measuring a known liquidwith 0 concentration of chlorine or monochloramine. The correctedanalytical signals for chlorine and monochloramine are determined bysubtracting the blank values from the measured values at each appliedpotential. The data points for the corrected analytical signals versusthe potential voltage are plotted in the graph shown in FIG. 40.

The following chart illustrates the ratios of the analytical signals forcorresponding ratios of the potentials:

Ratios of Corrected Analytical Signals (μA) versus ratios of potentials(V) Ratios (V) Chlorine (corrected) NH₂Cl (corrected) 0.10:0.30 1.40 8.578 0.00:0.30 1.76 16.913

As can be seen from the foregoing data, it is possible to determinewhether the species is monochloramine or free chlorine by the ratio ofthe measurement of two potentials. A ratio of greater than 5 indicatesthat the species is monochloramine. A display icon can be used toindicate that monochloramine is present to thereby invite the use toperform an action. For example, when used with a swimming pool, the iconmay warn the operator that the water needs to be superchlorinated orthat a non-chlorine shock compound needs to be added to lower themonochloramine level.

The data also illustrates the dramatic difference in signals produced bysimilar levels of free chlorine and monochloramine. Two storedcalibrations can be used—one for free chlorine and one for totalchlorine. The ratio can also be used to quantify the monochloramine andfree chlorine fractions based on the magnitude of the ratio. The storedcalibrations can be adjusted to read out in parts per million (ppm) ofmonochloramine if monochloramine is present. In a drinking waterapplication, the residual displayed can be adjusted to display ppmmonochloramine instead of free chlorine to more accurately display thechlorine level. Otherwise, the sensor system may drastically underreport the sanitizer levels due to the lower signal response ofmonochloramine.

SUMMARY OF THE INVENTION

An amperometric system with low maintenance requirements is needed. Thesystem should be able to operate for extended periods unattended. Thesystem should operate for extended periods (e.g., for up to a year)without a sensor replacement or other maintenance. Preferably, thechlorine sensor and the pH sensor should be replaceable individually toreduce the cost of operation. Preferably, the sensor incorporates amethod of overcoming the polarization effects of cyanuric acid (CYA).The sensor should not require frequent recalibration, and if a sensor isremoved or replaced, a sensor should quickly stabilize and reportreliable readings.

An aspect of embodiments in accordance with the present invention is asensor system that provides a reliable sensor platform that has a longlived chlorine sensor, provides a replaceable pH sensor and requiresonly infrequent calibration. The sensor system provides fast measurementresponse times, is largely unaffected by changes in flow, pH,temperature or conductivity, and is capable of direct insertion in apipe. The sensor is resistant to biofouling and to high levels of waterhardness, which makes the sensor practical for use in the mostchallenging applications including seawater and wastewater as well as incleaner applications for the measurement of chlorine in drinking water.

The sensor system disclosed herein can be used with either a flow cellthat receives a constant flow rate of water or an integrated pumpversion that can be operated independent of the flow rate and is capableof direct pipe insertion.

An embodiment of the sensor system includes cleaning balls, which aremoved by the flow from either the pump or the water supplied to the flowcell to abrade the surface of the electrodes and the pH sensor to removesalts that build up as a result of water hardness and in some casesoxide formation. Preferably, the sensor system usespolytetrafluoroethylene (PTFE) (e.g., Teflon® resin) balls or otherpolymeric balls to simultaneously clean scale from both the pH and thereference electrodes and from the auxiliary electrode and the workingelectrode for the chlorine sensor. In some cases, it is preferable touse PEEK (polyether ether ketone) balls in a seawater oxidant sensor.The flow cell is advantageously designed so that the balls cannot escapeduring normal operation. An inlet port is substantially smaller than theball diameter, which prevents ball egress when the flow is discontinuedand sensor is removed. A small clearance between the sensor and thewalls of the flow cell prevent balls from escaping via the outlet portof the flow cell. The housing is configured so that captured air can bepurged during operation. In particular, the outlet port is positionedabove the sensor end when the sensor system is inserted into thehorizontally oriented pipe fitting. The position of the outlet portenables air to egress the flow cell.

In accordance with another embodiment of a self-cleaning sensor havingan integral pump, the sensor provides a consistent reading with varyingrates of flow from 0 to 7 feet per second (FPS). The sensor isself-cleaning under challenging conditions. The sensor provides a highersensitivity signal due to the higher velocity across the electrodesdeveloped within the sensor cover by the integral pump. The integralpump provides a consistent flow across the chlorine electrodes. Thesensor cleans all three electrodes simultaneously. The sensor alsocleans the pH sensor glass. The sensor is integrated to provideautomatic pH compensation. The sensor has long-life electrodes.

In the sensor, a magnetically coupled impeller is rotated by a motorthat produces a water flow across the electrodes with a generallyconstant flow velocity. This flow rate also improves the electrodesensitivity to chlorine with higher velocity across the electrodes. Acover retains the balls and forms a pump volute to produce the flow andmove the balls to abrade the electrode surfaces and interior portion ofthe sensor to remove scale and biological coatings. In a preferredembodiment, the motor is a 3-phase brushless DC (BLDC) motor that haslow noise and that requires low current. The pump is a self-priming. Thepump includes an integrated tachometer to detect motor movement and tocontrol motor speed.

In preferred embodiments, the wetted portions of the sensor that aresubject to biofouling are manufactured from PTFE or Ultra High MolecularWeight Polyethylene (UHMW) (e.g., Teflon® resin) and are resistant toadhesion of organic gels and microorganisms. The sensor has ahemispherical shape, which allows the release of large particulates whenthe sensor is installed in a plumbing tee and subject to a flowingstream across the immersed sensor.

In preferred embodiments of the sensor, a negative potential is appliedto the electrodes subsequent to the application of the measurementpotential. The application of the negative potential provides anunanticipated benefit of preventing the passivation of the electrodewhen cyanuric acid (CYA) is present. In the absence of the subsequentlyapplied negative potential, the electrode is rapidly passivated suchthat within approximately 24 hours, the signal from the sensor drops byapproximately 90% and thus no longer correlates with the chlorine levelbeing measured. The subsequently applied negative potential minimizesthe effects of polarization by preventing the gradual loss ofsensitivity, which enables indefinitely repeatable measurements.

The sensor system disclosed herein includes a long-life chlorine sensor,which may have different configurations. In one advantageousconfiguration, the chlorine sensor comprises a platinum workingelectrode and a platinum auxiliary electrode. In another advantageousconfiguration, the chlorine sensor comprises a platinum auxiliaryelectrode and a gold working electrode. The platinum electrode maycomprise an alloy of platinum. The gold electrode may comprise an alloyof gold. In another configuration, both electrodes are platinum. Inanother configuration, both electrodes are gold. Preferably, thechlorine sensor is enclosed within a housing comprising polyether etherketone (PEEK), a semicrystalline thermoplastic having excellentmechanical properties, including hydrophobicity and chemical resistanceproperties. Alternatively UHMW may be used.

When using two platinum electrodes to measure seawater or water inswimming pools having cyanuric acid, a measurement potential ofapproximately 0.25 volt is applied for approximately 30 seconds. Themeasurement potential is followed by a potential of −2.0 volts appliedfor approximately 5 seconds. When using gold electrodes to measuredrinking water and waste water, a measurement potential of approximately0.25 volt is applied for approximately 5 to 30 seconds, followed by apotential of −0.6 volts applied for approximately 1 to 10 seconds. Whenusing platinum electrodes to measure drinking water and waste water, ameasurement potential of approximately 0.25 volt to approximately 0.4volts is applied for approximately 5 to 30 seconds, followed by apotential of −0.6 volts applied for approximately 1 to 5 seconds. Theresults of measurements using the sensor disclosed herein and theresults of DPD spectrophotometric measurements are comparable.

Since the same sensor may be used in several different applications withdifferent water make up, certain parameter settings such as measurementtime and potentials may be optimized for each water type. With one menuselection, the user can select the water type and the parameter settingsare changed. This feature simplifies operation and facilitates use ofthe sensor in different applications with the single sensor optimizedfor a certain water type. For example, in one embodiment, “seawater” canbe selected or “drinking water” can be selected.

The sensor sequentially measures several water parameters in addition tochlorine, including ORP, pH and conductivity. The ORP measurement can beperformed in a very short interval of 5 seconds. The cycling betweenmeasurements impresses a potential that prevents absorption of organicson the electrode surface. To speed up equilibration of the ORPmeasurement, the potential is set to 0.0 Volts before the start of theORP measurement. This enables rapid, reliable ORP measurements inchallenging water conditions.

The sensor system disclosed herein includes a replaceable pH sensor,which may be a single-junction sensor or a double-junction sensor. Inpreferred embodiments, the pH sensor is a double-junction sensor or adifferential style pH sensor. Alternatively, a solid-state referenceelectrode may be substituted for the pH sensor in applications where pHmeasurement is unnecessary. For example, the pH of seawater is fairlyconstant and generally does not need to be measured. The use of thesolid reference electrode eliminates the fragile glass bulb requiredwith most pH sensors and enables long life and low maintenancerequirements. Each of the reference/pH cartridges may used with only asingle change made by the user with the software interface.

The sensor system disclosed herein includes a pump having a volutecover. The pump includes an impeller that moves a plurality of cleaningballs that remove scale and other contaminants from the electrodes.Preferably, the cleaning balls comprise Teflon® resin; however, thecleaning balls may comprise ceramic, glass or other suitable materials.The impeller is coupled to a motor by diametrically magnetized magnets.In certain preferred embodiments, the diametrically magnetized magnetsare neodymium magnets.

Preferably, the sensor system disclosed herein includes an integratedtemperature sensor and a memory device on a printed circuit board (PCB).In certain embodiments, the temperature sensor is potted into the sensorhousing a thermally conductive epoxy. In particularly preferredembodiments, the sensor body wall has a thickness of approximately 0.03inch to minimize the thermal time constant from the fluid being measuredto the temperature system. The memory device, which is preferably anelectrically erasable programmable read only memory (EEPROM), storescalibration values for the sensor system and the serial number of thesensor system.

An aspect in accordance with embodiments disclosed herein is a sensorsystem for measuring at least one parameter of water. The sensor systemcomprises an electronics subsystem and a sensor housing. The sensorhousing is electrically and mechanically coupled to the electronicssubsystem. The sensor housing comprises a chamber that receives watervia at least one inlet and that releases water via at least one outlet.At least one sensor within the sensor housing has at least one electrodeexposed to water in the chamber. A flow generator causes water to flowthrough the chamber.

Preferably, the sensor housing is configured to be inserted into a pipecarrying the water for which the parameter is to be measured.

Preferably, the flow generator is a pump that comprises a motor and animpeller. The sensor housing comprises a wet side exposed to water and adry side isolated from water. The motor is mounted on the dry side ofthe sensor housing; and the impeller is mounted on the wet side of thesensor housing. The impeller is magnetically coupled to the motor suchthat rotation of the motor on the dry side of the housing rotates theimpeller on the wet side of the housing. In certain embodiments, themotor comprises a brushless DC motor.

In an illustrated embodiment, the sensor system is an amperometricsensor system.

Preferably, the sensor system further comprises a plurality of movableobjects constrained within the chamber and movable by water flowing inthe chamber to impinge upon and clean a surface of the at least oneelectrode. For example, the movable objects may comprise glass spheresor may comprise polytetrafluoroethylene (PTFE) spheres. Preferably, themovable objects have dimensions sufficiently larger than the at leastone inlet and the at least one outlet such that the movable objects areconfined to the chamber.

In certain embodiments of the system, the at least one sensor comprisesat least three electrodes, and the at least three electrodes are cleanedsimultaneously by a plurality of movable objects moving within thechamber as water flows through the chamber.

In certain embodiments of the system, the at least one electrodecomprises the electrodes in a chlorine sensor and the electrodes in a pHsensor wherein the electrodes in the chlorine sensor and the electrodesin the pH sensor are cleaned simultaneously. Preferably, the at leastthree electrodes are cleaned simultaneously by a plurality of movableobjects within the chamber as water flows through the chamber.

In certain embodiments of the system, the at least one outlet of thechamber is positioned so that any air in the chamber exits through theat least one outlet.

Another aspect in accordance with embodiments disclosed herein is anamperometric sensor system for measuring at least one parameter ofwater. The sensor comprises at least one sensor probe positioned influid communication with water having the parameter to be measured. Theprobe comprises a plurality of electrodes. The sensor generates anoutput signal responsive to the concentration of the parameter to bemeasured. A control system electrically coupled to the sensor probeapplies differential voltages between at least a first electrode of theplurality of electrodes and a second electrode of the plurality ofelectrodes. The control system is configured to generate a firstdifferential measuring voltage in a range between −0.2 volt and +0.5volt between the first electrode and the second electrode, and togenerate a second differential measuring voltage between 0 volt and −5volts between the first electrode and the second electrode. The seconddifferential measuring voltage is applied for a duration of at least 0.1second following the first differential measuring potential.

In certain embodiments of the system, the first electrode and the secondelectrode each comprise platinum. Preferably, the first electrode andthe second electrode are planar electrodes deposited on a nonconductivesubstrate. For example, the sensor system is configured for use withwater that includes Cyanuric acid.

In certain embodiments of the system, at least one of the firstelectrode and the second electrode comprises gold.

In certain embodiments of the system, the sensor system is configuredfor use with water comprises seawater.

In certain embodiments of the system, the sensor system is configuredfor use with water includes greater than 1,000 parts per million (ppm)of sodium chloride.

Another aspect in accordance with embodiments disclosed herein is amethod for quantifying two species in water. The method comprisesapplying a first differential measurement potential between at leastfirst and second electrodes of at least one sensor probe; and measuringa first output signal responsive to the concentration of a parameter ofwater to be measured and responsive to the first differentialmeasurement potential. The method further comprises applying a seconddifferential measurement potential between the at least first and secondelectrodes of the at least one sensor probe, where the seconddifferential measurement potential is different from the firstdifferential measurement potential. The method further includesmeasuring a second output signal responsive to the concentration of theparameter of water to be measured and responsive to the seconddifferential measurement potential. The method further includesdetermining a ratio of the first output signal to the second outputsignal; and determining which of the two species is present based on theratio of the first output signal to the second output signal. The methodthen calculates and quantifies the species determined to be present. Incertain embodiments of the method, when both species are present, themethod quantifies each species according to the ratio of the firstoutput signal to the second output signal. In certain aspects of themethod, the first species comprises free chlorine and the second speciescomprises chloramine; and the ratio of the first output signal to thesecond output signal has a value in a first range when the species inthe water comprises free chlorine and has a value in a second range whenthe species in the water comprises chloramine.

Another aspect in accordance with embodiments disclosed herein is amulti-use sensor system for sensing pH in water. The sensor systemcomprises an electronics base unit that houses electronics circuitrycommon to a plurality of sensor applications. The sensor system furthercomprises a removable sensor cartridge that houses at least onereplaceable pH sensor, wherein the replaceable pH sensor in theremovable sensor cartridge is configured to be one of (1) a differentialpH sensor, (2) a combination pH sensor and reference electrode, or (3) areference only sensor.

In certain embodiments of the multi-use sensor system, the replaceablepH sensor has a first sensing end having a flared body portion and has asecond connecting end having a sensor connector fixed thereon. Thereplaceable pH sensor has a fixed length between the flared body portionand the sensor connector. The removable sensor cartridge includes aflared cavity configured to receive the flared body portion of thereplaceable pH sensor and has a mating connector configured to mate withthe sensor connector. The mating connector is positioned to electricallyand mechanically engage the sensor connector when the flared bodyportion of the replaceable pH sensor is positioned within the flaredcavity.

Another aspect in accordance with embodiments disclosed herein is amethod for preventing polarization of electrodes when measuringoxidation reduction potential (ORP) using the electrodes. The methodcomprises impressing a voltage potential between two electrodes of asensor for a first predetermined time. The method further comprisesremoving the voltage potential between the two electrodes of the sensorfor a second predetermined time and measuring ORP. The method furthercomprises repeating the impressing and removing actions for respectivefirst predetermined times and second predetermined times, whereinimpressing the voltage potential and removing the voltage potentialprevents polarization of the electrodes.

In certain embodiments of the method, a total time of the firstpredetermined time followed by the second predetermined time is lessthan one minute.

Another aspect in accordance with embodiments disclosed herein is areconfigurable amperometric sensor having first and second electrodes tomeasure at least one parameter of water, wherein a type of water inwhich the parameter is measured can be one of drinking water, seawaterand pool water. The amperometric sensor comprises a selector forselecting the type of water in which the at least one parameter of thewater is to be measured. A means responsive to the selector configuresthe amperometric sensor in accordance with the type of water selected tovary at least one operating parameter in accordance with a preset set ofoperating parameters for each type of water. The operating parametersinclude a measurement time during which a first voltage potential isapplied between the first and second electrodes to measure the at leastone parameter of the water and further include a magnitude of a secondvoltage potential applied by the sensor during a non-measurement time toprevent polarization caused by the first voltage potential.

Another aspect in accordance with embodiments disclosed herein is amethod of cleaning an amperometric sensor having a plurality ofelectrodes positioned on the face of a sensor body. Each of theplurality of electrodes has a surface exposed to water having at leastone parameter to be measured by the plurality of electrodes. The methodcomprises confining a plurality of movable objects in a cavity thatcomprises a volume of water exposed to the surfaces of the plurality ofelectrodes. The method further comprises flowing water into, through andout of the cavity to cause the water to flow across the surfaces of theplurality of electrodes. The method further comprises circulating theplurality of movable objects within the cavity to cause the plurality ofmovable objects to impinge on the surfaces of the plurality ofelectrodes as the water flows through the cavity. The plurality ofmovable objects abrade the surfaces of the plurality of electrodes tothereby clean the electrodes.

In certain embodiments of the method, the movable objects are spherical.In certain embodiments of the method, the movable objects compriseglass. In certain embodiments of the method, the movable objectscomprise polytetrafluoroethylene (PTFE).

In certain embodiments of the method, a control system electricallycoupled to the sensor probe applies differential voltages between atleast a first electrode of the plurality of electrodes and a secondelectrode of the plurality of electrodes. Preferably, the control systemgenerates a first differential measuring voltage in a range between −0.2volt and +0.5 volt between the first electrode and the second electrode,and generates a second differential voltage between 0 volt and −5.0volts between the first electrode and the second electrode. The seconddifferential voltage is applied for a duration of at least 0.1 secondfollowing the first differential measuring potential.

In certain embodiments of the method, the water is seawater and theoxidant levels measured are between 1 and 500 ppm.

Another aspect in accordance with embodiments disclosed herein is amethod of prevention of passivation in an amperometric sensor system.The method comprises positioning at least one sensor probe in fluidcommunication with water having a parameter to be measured. The probecomprises a plurality of electrodes. The sensor generates an outputsignal responsive to the concentration of the parameter to be measured.The method further comprise electrically coupling a control system tothe sensor probe to apply differential voltages between at least a firstelectrode of the plurality of electrodes and a second electrode of theplurality of electrodes. The control system is configured to generate afirst differential measuring voltage in a range between −1.0 volt and+0.5 volt between the first electrode and the second electrode, and togenerate a second differential voltage between 0 volt and −5.0 voltsbetween the first electrode and the second electrode. The seconddifferential voltage is applied for a duration of at least 0.1 secondfollowing the first differential measuring potential.

In certain embodiments of the method, the water includes cyanuric acid.

In certain embodiments of the method, the water is seawater, and theparameter to be measured is an oxidant in the seawater. In suchembodiments, the plurality of electrodes includes a solid-statereference electrode and the surface of at least one of the electrodes iscoated with platinum.

Another aspect in accordance with embodiments disclosed herein is amethod of operating an amperometric sensor having a plurality ofelectrodes positioned on the face of a sensor body. Each of theplurality of electrodes has a surface exposed to water having at leastone parameter to be measured by the plurality of electrodes. The methodcomprises positioning the surfaces of the plurality of electrodes in acavity that comprises a volume of water. The method further comprisesoperating a flow generator to flow the water having the at least oneparameter to be measured into, through and out of the cavity to causethe water to flow across the surfaces of the plurality of electrodes ata substantially constant velocity. The method further comprisesmeasuring the at least one parameter of the water while the water isflowing across the surfaces of the plurality of electrodes.

In certain embodiments of the method, the flow generator comprises animpeller and comprises a motor to impart rotation to the impeller. Themethod further comprises positioning the impeller in the cavity toexpose the impeller to the water in the cavity, and positioning themotor outside the cavity in a location isolated from the water. Themotor is mechanically coupled to a first rotatable coupling deviceoutside the cavity. Energy is applied to the motor to rotate the firstrotatable coupling device. The first rotatable coupling device ismagnetically coupled to the impeller within the cavity to cause theimpeller to rotate within the cavity. In the illustrated embodiment, themotor has a motor torque. The first rotatable coupling device is coupledto the impeller with at least a minimum coupling force, wherein theminimum coupling force selected to be greater than the motor torque sothat the motor will not rotate the first coupling device if the impellercannot rotate.

Another aspect in accordance with embodiments disclosed herein is amethod for reducing build-up of contamination on a sensor. The methodcomprises positioning a surface of a sensor in an enclosure having atleast one inlet to allow water to enter the enclosure and having atleast two outlets to allow water to exit the enclosure. The enclosurecontains a plurality of movable particles. The movable particles havedimensions selected to prevent the movable particles from passing out ofthe enclosure via the outlets. The method further comprises flowingwater within the enclosure from the inlet to the outlets to cause thewater to flow over the surface of the sensor. The flow of the watercauses at least some of the movable particles to impinge on the surfaceof the sensor to dislodge contamination from the surface of the sensor.The outlets are oriented with respect to a direction of flow of thewater to inhibit the movable particles from blocking the at least twooutlets.

Another aspect in accordance with embodiments disclosed herein is anapparatus. The apparatus comprises an enclosure that houses a surface ofa sensor adapted to measure a characteristic of water. The enclosure hasat least one inlet to allow water to enter the enclosure and has atleast two outlets to allow water to exit the enclosure. The enclosurecontains a plurality of movable particles. The movable particles havedimensions selected to prevent the movable particles from passing out ofthe enclosure via the outlets. The apparatus further comprise a flowgenerator that produces a flow of water within the enclosure from theinlet to the outlets to cause the water to flow over the surface of thesensor. The flow of the water causes at least some of the movableparticles to impinge on the surface of the sensor to inhibit build-up ofcontaminates on the surface. The outlets of the enclosure are orientedwith respect to the flow of the water to inhibit the movable particlesfrom blocking the outlets.

In certain embodiments of the apparatus, the enclosure comprises aninner cavity configured to constrain the movable particles in a volumeproximate to the surface of the sensor. Preferably, the inner cavity hasa shape selected to cause the movable particles to move in a circulatingpattern within the inner cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments in accordance with aspects of the present invention aredescribed below in connection with the attached drawings in which:

FIG. 1 illustrates a perspective view of an embodiment of a sensorsystem as viewed from the proximal end of the sensor system;

FIG. 2 illustrates a perspective view of an embodiment of the sensorsystem of FIG. 1 as viewed from the distal end of the sensor system;

FIG. 3 illustrates a right side elevational view of the sensor system ofFIG. 1;

FIG. 4 illustrates a perspective view of the sensor system of FIG. 1when engaged with a pipe fitting in a water system;

FIG. 5 illustrates a right elevational view of the sensor system and thepipe fitting of FIG. 4 showing the sensor housing of the sensor systemextending into the pipe fitting;

FIG. 6 illustrates an exploded perspective view of the sensor system ofFIG. 1 viewed from the proximal end of the sensor system;

FIG. 7 illustrates an exploded perspective view of the sensor system ofFIG. 1 viewed from the distal end of the sensor system;

FIG. 8 illustrates an enlarged exploded perspective view of the sensorhousing of the sensor system of FIG. 1 viewed from the proximal end ofthe sensor housing;

FIG. 9 illustrates an enlarged exploded perspective view of the sensorhousing of the sensor system of FIG. 1 viewed from the distal end of thesensor housing;

FIG. 10 illustrates a perspective view of the pH sensor of the sensorhousing of FIGS. 8 and 9 viewed from the distal end of the pH sensor;

FIG. 11 illustrates a perspective view of the chlorine sensor of thesensor housing of FIGS. 8 and 9 viewed from the distal end of thechlorine sensor;

FIG. 12 illustrates a perspective view of the temperature sensor of thesensor housing of FIGS. 8 and 9 viewed from the distal end of thetemperature sensor;

FIG. 13 illustrates an elevational view of the distal end of the sensorbody of FIGS. 8 and 9 before adding components to the sensor body;

FIG. 14 illustrates an elevational view of the proximal end of thesensor body of FIGS. 8 and 9 before adding components to the sensorbody;

FIG. 15 illustrates a cross-sectional perspective view of the sensorbody of FIGS. 8 and 9 as viewed from the proximal end, the cross sectiontaken along the line 15-15 in FIG. 14 to pass through the centers of thebores that receive the motor and the impeller;

FIG. 16 illustrates a cross-sectional perspective view of the sensorbody of FIGS. 8 and 9 as viewed from the proximal end, the cross sectiontaken along the line 16-16 in FIG. 14 to pass through the approximatecenter of the bore that receives the chlorine sensor;

FIG. 17 illustrates an exploded perspective view of the motor assemblyand the impeller assembly of the sensor housing as viewed from thedistal end of the sensor system;

FIG. 18 illustrates an exploded perspective view of the impellerassembly of FIG. 17 as viewed from the proximal end of the impellerassembly;

FIG. 19 illustrates a cross-sectional elevational view of the assembledsensor housing of FIG. 9 without the sensor cover, the cross sectiontaken along the line 19-19 in FIG. 9 to pass through the approximatecenter of the sensor body;

FIG. 20 illustrates a perspective view of the cover of the sensorhousing of FIGS. 8 and 9 viewed from the proximal end of the sensorcover;

FIG. 21 illustrates a perspective view of the cover of the sensorhousing of FIGS. 8 and 9 viewed from the distal end of the sensor cover;

FIG. 22 illustrates an elevational view of the proximal end of cover ofthe sensor housing of FIGS. 8 and 9;

FIG. 23 illustrates a perspective cross-sectional view of the cover thesensor housing of FIG. 22 taken along the line 23-23 in FIG. 20;

FIG. 24 illustrates an elevational view of the distal end of the sensorhousing with the housing cover illustrated in phantom to show thecomponents within the housing cover;

FIG. 25 illustrates an exploded perspective view of the panel andprinted circuit board of the electronics enclosure of the sensor systemof FIG. 1 viewed from the proximal surface of the panel;

FIG. 26 illustrates an exploded perspective view of the panel andcircuit board of the electronics enclosure of the sensor system of FIG.1 viewed from the distal surface of the printed circuit board;

FIG. 27 illustrates a block diagram of the electronics system of thesensor system of FIG. 1;

FIG. 28 illustrates a timing diagram for the differential voltageapplied to the chlorine sensor when the sensor system of FIG. 1 is usedto measure sea water or to measure the water in a chlorinated swimmingpool;

FIG. 29 illustrates a timing diagram for the differential voltageapplied to the chlorine sensor when the sensor system of FIG. 1 is usedto measure drinking water;

FIG. 30 illustrates a perspective view of a further embodiment of asensor system as viewed from the proximal end of the sensor system;

FIG. 31 illustrates a perspective view of the sensor system of FIG. 30as viewed from the distal end of the sensor system;

FIG. 32 illustrates an exploded perspective view of the sensor system ofFIG. 30 viewed from the proximal end of the sensor system;

FIG. 33 illustrates an exploded perspective view of the sensor system ofFIG. 30 viewed from the distal end of the sensor system;

FIG. 34 illustrates an exploded perspective view of the electronicshousing of FIGS. 32 and 33 viewed from the proximal end of theelectronics housing;

FIG. 35 illustrates an exploded perspective view of the electronicshousing of FIGS. 32 and 33 viewed from the distal end of the electronicshousing

FIG. 36 illustrates an exploded perspective view of the sensor housingof FIGS. 32 and 33 viewed from the proximal end of the sensor housing;

FIG. 37 illustrates an exploded perspective view of the sensor housingof FIGS. 32 and 33 viewed from the distal end of the sensor housing;

FIG. 38 illustrates a perspective view of the pH/reference probeincorporated into the sensor housing of FIGS. 32 and 33 viewed from theproximal end of the pH/reference probe;

FIG. 39 illustrates a perspective view of the pH/reference probeincorporated into the sensor housing of FIGS. 32 and 33 viewed from thedistal end of the pH/reference probe; and

FIG. 40 illustrates a graph of sensor current (I) in microamperes versusapplied potential (V) for chorine and for NH₂Cl; and

FIG. 41 illustrates a flow chart of a method of quantifying two speciesin water implemented by the sensor system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The amperometric sensor is disclosed herein with respect to exemplaryembodiments. The embodiments are disclosed for illustration of thesensor system and are not limiting except as defined in the appendedclaims.

FIG. 1 illustrates a perspective view of an embodiment of a sensorsystem 100 in accordance with aspects of the present invention. Thesensor system comprises an electronics enclosure 110, an intermediatehousing 112, a sensor housing 114 and a communications cable 116. Thecommunications cable is terminated at a free end with a connector 118.The opposite end of the communications cable enters the electronicsenclosure via a liquid-tight cordgrip 120. Wires within thecommunications cable are terminated on a printed circuit board (notshown in FIG. 1) within the electronics enclosure as described below.

FIG. 2 illustrates a perspective view of the sensor system 100 of FIG. 1viewed from the sensor end. FIG. 3 illustrates a right side elevationalview of the sensor system of FIG. 1. FIG. 4 illustrates a perspectiveview of the sensor system engaged with a tee pipe fitting 124. FIG. 5illustrates an end elevational view of the sensor system and pipefitting of FIG. 4 showing the sensor housing 114 within the pipe fittingin a position such that when water is flowing through the pipe fittingthe sensor housing will be immersed in the water. FIG. 6 illustrates anexploded perspective view corresponding to the view in FIG. 1. FIG. 7illustrates an exploded perspective view corresponding to the view inFIG. 2.

As used in the following description “proximal” refers to portions ofthe sensor system 100 and subassemblies towards the electronicsenclosure 110 (e.g., towards the left in FIG. 1) and “distal” refers toportions of the sensor system and subassemblies towards the sensorhousing 114 (e.g., towards the right in FIG. 1). Accordingly, in FIGS. 4and 5, for example, the electronics enclosure at the proximal end of thesensor system is at the left in each figure and the sensor housing atthe distal end of sensor system is positioned within the pipe fitting124 at the right in each figure.

The intermediate housing 112 of the sensor system 100 generallycorresponds to a cylindrical section 130 having a nominal inner diameterand a nominal outer diameter corresponding to the inner diameter and theouter diameter, respectively, of a 1½-inch pipe. The intermediatesection has an overall length of approximately 4½ inches between theproximal end and the distal end. A first end portion 132 of theintermediate housing at the distal end has a reduced outer diameter fora length of approximately 0.4 inch. An outer threaded portion 134adjacent the first end portion has male threads that correspond to themale threads of a 1½-inch pipe such that the first end portion and theouter threaded portion can be inserted into a conventional 1½-inch pipefitting (e.g., the fitting 124 in FIGS. 4 and 5) by engaging the malethreads of the intermediate housing with the female threads of thefitting.

As shown in FIG. 7, the inner surface of the first end portion 132 andthe inner surface of the outer threaded portion 134 have female threads136 formed therein that are compatible with a 1¼-inch pipe fitting. Thefemale threads of the intermediate housing receive male threads 138 ofthe sensor housing 114 when the sensor housing is attached to theintermediate housing as shown in FIGS. 1-5. A second end portion 140 ofthe intermediate housing at the proximal end has a reduced outerdiameter and is sized to fit snugly within a circular opening 142 (FIG.7) in the distal surface of the electronics enclosure 110.

In the illustrated preferred embodiment, the intermediate housing 112further includes a first flat portion 144 and a second flat portion 146that are diametrically opposed from each other along the central body130 of the intermediate housing. The two flat portions are engageable bya wrench (not shown) so that the intermediate housing can be easilyrotated to engage the outer male threads of the intermediate housingwith the female threads of the pipe fitting shown in FIGS. 4 and 5.

As shown in the exploded perspective view of the sensor system 100 inFIGS. 6 and 7, the intermediate housing 112 is a conduit forinterconnection cables and other equipment that extend from the sensorhousing 114 to the electronics enclosure 110.

As shown in the enlarged exploded perspective views of the sensorhousing 114 in FIGS. 8 and 9, the sensor housing comprises a sensor body150 and a sensor cover 152. The sensor body has a proximal surface 154and a distal surface 156. The sensor cover is positioned over the distalsurface of the sensor body.

A pH sensor unit 160 (shown in more detail in FIG. 10) extends throughthe sensor body 150 from the distal surface 156 of the sensor body andprojects from the proximal surface 154 of the sensor body into theintermediate housing 112 when the sensor system is assembled as show inFIGS. 1-5. A pH sensor cable 162 extends from the proximal end of the pHsensor unit to a connector 164. Preferably, the pH sensor cable is ashielded cable. The connector engages a mating connector on a printedcircuit board (described below) within the electronics enclosure 110.The pH sensor provides a pH signal on one wire of the pH signal cableand a common reference signal on another wire of the pH signal cable. Anactive distal end 166 of the pH sensor is exposed at the distal surfaceof the sensor body. Preferably, the pH sensor is a replaceable pHsensor, which may be a single-junction sensor or a double-junctionsensor. In preferred embodiments, the pH sensor is a double-junctionsensor.

A chlorine sensor 170 (shown in more detail in FIG. 11) extends throughthe sensor body 150 and extends from the distal surface 156 of thesensor body toward the proximal surface 154 of the sensor body. Achlorine sensor cable 172 extends from the proximal end of the chlorinesensor and passes through the intermediate housing 112. The chlorinesensor cable comprises a pair of chlorine sensor wires. A respectiveconnector 174 at the proximal end of each of chlorine sensor wiresengages a respective mating connector on the printed circuit boardwithin the electronics enclosure 110. An active distal end 176 of thechlorine sensor is exposed at the distal surface of the sensor body. Theactive distal end of the chlorine sensor comprises a pair of electrodes178. One of the electrodes is a working electrode, and the otherelectrode is an auxiliary electrode. A respective one of the electrodesis connected to a respective one of the chlorine sensor wires.

The chlorine sensor 170 is a long-life chlorine sensor, which may havedifferent configurations. In one advantageous configuration, thechlorine sensor comprises a platinum working electrode and a platinumauxiliary electrode. In another advantageous configuration, the chlorinesensor comprises a platinum auxiliary electrode and a gold workingelectrode. The platinum electrode may comprise an alloy of platinum. Thegold electrode may comprise an alloy of gold. In another configuration,both electrodes are platinum. In another configuration, both electrodesare gold. Preferably, the chlorine sensor is enclosed within a housingcomprising polyether ether ketone (PEEK), a semicrystallinethermoplastic having excellent mechanical properties and chemicalresistance properties. Alternatively, the platinum metal may bedeposited on a non-conductive water resistant substrate (such as ceramicPCB material) by plating, vapor deposition or other means to form aconcentric ring and a disk. A conductive path is formed using a selectedmetal, such as silver, using a printed circuit board process known as“via-in-pad, conductive silver via plug with planarized surface copper.”This process developed by the Via Protection Task Group (D-33d) of theRigid Printed Circuit Board Committee (D-30) of the IPC (IPC, 3000Lakeside Drive, Suite 309S, Bannockburn, Ill. 60015-1219) provides aconductive pad without producing any holes in the printed circuit board,thus facilitating sealing and waterproofing.

A temperature sensor 180 (shown in more detail in FIG. 12) is embeddedwithin the sensor body 150. A temperature sensor cable 182 extends fromthe proximal end of the temperature sensor and passes out from theproximal surface 154 of the sensor body and then through theintermediate housing 112. A connector 184 on the proximal end of thetemperature sensor cable engages a mating connector on the printedcircuit board within the electronics enclosure 110. In the illustratedembodiment, the temperature sensor cable comprises a plurality of signalwires (e.g., 4 signal wires), and the connector comprises acorresponding plurality of connector elements. In the illustratedembodiment, the temperature sensor comprises a printed circuit board 186onto which are mounted components that generate signals responsive tothe temperature of the components. Preferably, the printed circuit boardis surrounded by a cylindrical plug 188 (shown in phantom) thatcomprises a thermally conductive potting compound. Accordingly, thetemperature of the sensor body is communicated to the temperaturesensor.

A motor assembly 190 is mounted on the proximal surface 154 of thesensor body 150. The motor assembly comprises a motor 192 mounted to thesensor body via a motor assembly bracket 194. A motor power cable 196extends from the motor and through the intermediate housing 112. Aconnector 198 on the motor power cable engages a mating connector on theprinted circuit board.

The sensor body 150 prior to assembly is shown in FIGS. 13-16. As shownin the cross-sectional view in FIG. 15, the sensor body includes a firstthrough bore 200 that extends from the distal surface 156 to theproximal surface 154. The diameter of a first portion 202 of the firstthrough bore proximate the distal surface is larger than the diameter ofa second portion 204 of the first through bore proximate the proximalsurface so that the first through bore includes a taper 206 between thetwo portions. The pH sensor unit 160 (FIG. 10) includes a taper 210 thatrests against the taper of the sensor body when the pH sensor isinserted into the first through bore. An O-ring 212 around the pH sensorunit distal to the taper seals the first portion of the first throughbore to inhibit liquid flow through the first through bore. Ifnecessary, the pH sensor can be replaced by pushing the pH sensor outfrom the distal end of the sensor body and inserting a replacement pHsensor with the attached O-ring into the first through bore until thereplacement pH sensor seats against the taper.

As shown in the cross-sectional view in FIG. 16, the sensor body 150includes a second through bore 220. The second through bore has a firstlarger diameter portion 222 proximate the distal surface 156 and asecond smaller diameter portion 224 proximate the proximal surface 154.A taper 226 interconnects the two portions. The chlorine sensor 170(FIG. 11) includes a taper 230 that rests against the taper of thesensor body when the chlorine sensor is inserted into the second throughbore. An O-ring 232 proximal to the taper on the chlorine sensor sealsthe second through bore to inhibit liquid flow through the secondthrough bore. If necessary, the chlorine sensor can be replaced bypushing the chlorine sensor out from the distal end of the sensor bodyand inserting a replacement chlorine sensor with the attached O-ringinto the second through bore until the replacement chlorine sensor seatsagainst the taper.

As shown in the cross-sectional view in FIG. 15, the distal surface 152of the sensor body 150 includes a first distal blind bore 240 thatextends for a first selected distance into the sensor body. A secondarydistal blind bore 242 at the bottom of the first distal blind bore has asmaller diameter that extends a second distance further into the sensorbody. The secondary distal blind bore is concentric with respect to thefirst distal blind bore about a common axis 244.

As shown in the cross-sectional view in FIG. 15, the sensor body 150further includes a first proximal blind bore 250 that has a diametersized to receive the temperature sensor 180 (FIG. 12). As shown in FIG.12, the temperature sensor preferably is embedded in the cylindricalthermally conductive epoxy plug 188, which has an outer diametercorresponding to the diameter of the first proximal blind bore so thatthe temperature sensor fits snugly within the first proximal blind bore.Accordingly, the temperature of the sensor body is communicated to theprinted circuit board 186 of the temperature sensor via the plug.

The proximal surface 154 of the sensor body 150 includes a secondproximal blind bore 260 having a first portion 262 with a largerdiameter at the proximal surface and having a second portion 264 with asmaller diameter in the direction towards the distal surface 156. Aledge 266 is formed between the first portion and the second portion.The second proximal blind bore is concentric with respect to the commonaxis 244 and is thereby aligned with the first distal blind bore 240 andthe secondary distal blind bore 242. A distal portion 270 of the secondportion of the second proximal blind bore is annular so that a shell 272is formed around the first distal blind bore and the second distal blindbore. As shown in FIG. 15, the shell precludes the passage of liquidfrom the first distal blind bore and the secondary distal blind bore tothe first proximal blind bore.

The second proximal blind bore 260 of the sensor body 150 receives thecomponents of the motor assembly 190 shown in an exploded perspectiveview in FIG. 17. The motor assembly includes the electrically poweredmotor 192. In the illustrated embodiment, the motor comprises aNidec-17S brushless DC (BLDC) motor, which is commercially availablefrom Nidec America, Braintree, Mass. In certain embodiments, the motormay be equipped with an integrated tachometer. The motor is attached tothe motor support bracket 194 via a plurality of fasteners (e.g., 3screws) 270. The bracket is attached to the proximal surface 154 of thesensor body (FIGS. 13-16) via a plurality of fasteners (e.g., 2 screws)272 that engage a corresponding plurality of threaded bores 274 (FIG.14) in the proximal surface of the sensor body. The bracket is alignedwith the sensor body via a pair of alignment studs 276 that fit into acorresponding pair of alignment bores 278 (FIG. 14) in the proximalsurface of sensor body. When the bracket is aligned and secured, theposition of the bracket aligns the rotational axis of the motor with thecommon axis 244. An output shaft 280 of the motor extends into the firstportion 242 of the second proximal blind bore. A coupler 282 couples theoutput shaft to a ring magnet 284. Preferably, the ring magnet comprisesa neodymium magnet. The motor is driven by power provided by the powercable 196 (shown in FIGS. 8 and 9).

The first distal blind bore 240 receives the shaft 302 of an impellerassembly 300 shown in an exploded perspective view in FIG. 17 and alsoshown in FIG. 18. The impeller assembly comprises an impeller disk 304having a plurality of curved impeller blades 306 attached to the distalface. The shaft extends from the proximal face of the impeller disk. Theshaft comprises a distal shaft portion 310 having a cavity 312 (FIG. 18)and a proximal shaft portion 314 having a cavity 316. The cavities inthe two shaft portions receive a rod magnet 318. The two shaft portionsare then attached (e.g., with an adhesive) to secure the rod magnettherein. The two shaft portions of the impeller shaft have an outerdiameter that is slightly less than the inner diameter of the firstdistal blind bore 240. The impeller assembly further includes a firstaxial bearing 320 that extends distally from the center of the impellerblades. As show in FIG. 18, the proximal shaft portion further includesa second axial bearing 322 that extends proximally from the center ofthe proximal shaft portion. The second axial bearing is sized to fitwithin the secondary distal blind bore 242. The impeller may beconstructed as one seamless piece by injection molding the impeller andinsert molding the magnet such that it is fully encapsulated within theimpeller shaft.

As shown in the cross-sectional view in FIG. 19, when the motor assembly190 is secured to the proximal surface 154 of the sensor body 150, thering magnet 284 is positioned around the shell 272 formed around thefirst distal blind bore 240 and the secondary distal blind bore 242.When the shaft is fully inserted into the first distal blind bore, therod magnet 314 is positioned within the magnetic field of the ringmagnet 284 (e.g., the rod magnet is concentric with the ring magnet andgenerally laterally aligned with the ring magnet). Accordingly, when thering magnet is rotated by the motor 192, the rod magnet rotatessynchronously with the ring magnet and causes the impeller assembly 300to rotate. Thus, the motor rotates the impeller without requiring acontinuous mechanical contact that would require an opening through thesensor body 150. Accordingly, no O-rings or other seals are required toprevent fluid flow around a rotating shaft. Preferably, the motor has aknown maximum torque. In the illustrated embodiment, the magneticcoupling force between the ring magnet and the rod magnet is selected tobe greater than the maximum torque of the motor. Accordingly, if alocked impeller condition should occur, the torque of the motor would beinsufficient to rotate the ring magnet with respect to the immobile rodmagnet. Accordingly, neither the ring magnet nor the rod magnet will bedemagnetized by any relative rotational movement between the twomagnets.

It can be seen that the respective distal surfaces of the sensor body150 and the sensor housing 114 comprise a “wet side” of the sensorhousing. The proximal surfaces of the sensor body and the sensor housingare isolated from any water on the wet side and thereby comprise a “dryside” of the sensor housing. The motor 192 is mounted on the dry side ofthe sensor housing. The impeller assembly 300 is mounted on the wet sideof the sensor housing. The shell 272 surrounding the impeller shaft 302isolates the wet side and the dry side. The ring magnet 284 and the rodmagnet 318 provide magnetic coupling through the shell such thatrotation of the motor on the dry side of the housing rotates theimpeller on the wet side of the housing.

As shown above in FIGS. 8 and 9, for example, the distal surface 156 ofthe sensor body 150 of the sensor housing 114 is covered by the sensorcover 152. The sensor cover is configured to direct the flow of fluidsover the exposed distal end 166 of the pH sensor 160 and the exposeddistal end 176 of the chlorine sensor 170. As shown in more detail inFIGS. 20-24, the sensor cover has a proximal surface 400 and a distalsurface 402. A plurality of protrusions 410 (e.g., 3 protrusions) extendfrom the proximal surface of the sensor cover and engage a correspondingplurality of recesses 412 (FIG. 9) in the distal surface of the sensorbody 150 to align the sensor cover with the sensor body. A countersunkbore 414 extends from the distal surface to the proximal surface of thesensor cover. As shown in FIG. 9, the countersunk bore receives afastener (e.g., a screw) 416 that engages a blind bore 418 in the distalsurface of the sensor body (see FIG. 13) to secure the sensor cover tothe sensor body.

The proximal surface of the sensor cover 152 has a recessed fluidchannel 430 formed therein. The recessed fluid channel has a firstgenerally cylindrical portion 432, a second tapered portion 424 and athird generally oval-shaped (ovoidal) portion 436. The first fluidchannel portion is configured to receive the disk 304 and the blades 306of the impeller assembly 300 so that the disk and impeller blades areable to rotate freely within the first portion. The relationship betweenthe impeller and the first fluid channel portion is shown in the distalview in FIG. 24 wherein the sensor cover is shown as transparent and thefluid channel is shown in phantom lines. A first bore 450 extendsdistally from the first portion and is positioned and sized to receivethe axial extension 320 of the impeller assembly (shown in FIG. 17). Thefirst bore of the sensor cover functions as a distal bearing for theimpeller. A plurality of through bores 452 (e.g., 6 through bores) aredisposed about the first bore and provide fluid access from the distalsurface of the sensor cover into the first portion of the fluid channel.

The first portion 432 of the fluid channel 430 is coupled directly tothe wider entry to the tapered second portion 434 of the fluid channel.The second portion of the fluid channel tapers to a narrower crosssection at the entry to the third portion 436.

As shown in FIG. 24, the third portion 436 of the fluid channel 430 ispositioned to generally surround the distal end 166 of the pH sensor 160and the distal end 176 of the chlorine sensor 170. The fluid enteringthe third portion exits via a radial outlet channel 460 formed on theproximal surface of the sensor cover 152. The position of the radialoutlet channel is advantageous because when the sensor system 100 isinserted horizontally into a pipe fitting, as shown for example in FIGS.4 and 5, the radial outlet channel is oriented upwardly as shown so thatcaptured air can be purged during operation.

The fluid within the third portion 436 of the fluid channel 430 alsoexits via an L-shaped outlet channel 462, which has an entry across fromthe radial outlet channel 460. As shown in the cross-sectional view inFIG. 23, the L-shaped channel exits radially inwardly from the thirdportion of the fluid channel and then turns to exit perpendicularlyoutwardly from the distal surface 402 of the sensor cover.

When the impeller assembly 300 is caused to rotate by the motor 192,fluid from the fluid system being measured is drawn into the firstportion 432 of the fluid channel 430 via the plurality of through bores452 and is impelled outward by the impeller blades 306. The fluid exitsthe first portion via the tapering second portion 434 and enters theoval-shaped third portion 436. The fluid flows throughout the thirdportion and then exits via the two outlet channels 460, 462 to return tothe fluid system being measured. Accordingly, the distal end 166 of thepH sensor 160 and the distal end 176 of the chlorine sensor 170 arecontinually refreshed with fluid from the fluid system being measured.

As further shown in FIGS. 9 and 24, for example, the third portion 436of the fluid channel 430 has a plurality of movable objects (orparticles) 470 positioned therein. For example, in the illustratedembodiment, the movable objects are spherical and are referred to hereinas balls. It should be understood that the movable objects may haveother shapes. In the illustrated embodiment, the balls move throughoutthe third portion in response to the fluid created by the impellerassembly 300 in the first portion 432 of the fluid channel. In theillustrated embodiment, the balls advantageously comprisepolytetrafluoroethylene (PTFE) (e.g., Teflon® resin) balls, glass ballsor balls of other suitable material. In some cases, it is preferable touse PEEK balls in a seawater oxidant sensor. For example, in oneembodiment, the balls have diameters of approximately 0.125 inch. Themovement of the balls in response to the moving fluid simultaneouslycleans scale and other contaminants from both the distal end 166 of thepH sensor 160 and from the distal end the electrodes at the distal end176 of the chlorine sensor 170.

The dimensions of the movable objects 470 (e.g., the diameters of theballs in the illustrated embodiment) are sufficiently large with respectto the second portion 434 and with respect to the outlet channels 460,462, that the movable objects (e.g., the balls) are prevented enteringthe narrow end of the second portion 434 of the fluid channel and fromexiting via either of the outlet channels. As shown in FIG. 23, forexample, the third portion of the fluid channel is deeper proximate tothe inlet from the second portion of the fluid channel to cause theballs to return to the vicinity of the inlet and be recirculated withinthe generally oval-shaped third portion 436 of the fluid channel overthe distal ends of the pH sensor and the chlorine sensor. In analternative embodiment (not shown), the third portion of the fluidchannel may be configured so that the balls are constrained to circulateonly in the vicinity of the pH sensor so that only the pH sensor isaffected by the cleaning action of the balls.

FIG. 25 illustrates an exploded perspective view of the respectiveproximal surfaces of a display and switch panel 500 and a printedcircuit board 510 within the electronics enclosure 110. FIG. 26illustrates an exploded perspective view of the respective distalsurfaces of the display and switch panel and the printed circuit board.The proximal surface of the display and switch panel includes a displaywindow 520 and a set 522 of membrane switches (described below). Thedistal surface of the display and switch panel includes a flexible cable524 connected to the set of membrane switches terminated in a connector526. The proximal surface of the printed circuit board supports anliquid crystal display (LCD) 530, which is aligned with the displaywindow of the display and switch panel. The proximal surface of theprinted circuit board further includes a connector 532 that engages theconnector from the display and switch panel to electrically connect theset of membrane switches to the printed circuit board.

The distal surface of the printed circuit board 510 includes a firstconnector 550 that engages the connector 164 on the pH sensor cable 162.A second connector 552 engages the two connectors 174 on the chlorinesensor cable 172. A third connector 554 receives the connectors 184 onthe temperature sensor cable 182. A fourth connector 556 engages theconnector 198 on the motor power cable 196. A fifth connector 560engages a connector on the cable 116 (FIGS. 1-5). For example, in oneembodiment the fifth connector comprises two contacts for power andground and two contacts the data communications signals.

A processor 600 on the distal surface of the printed circuit board 510controls the motor 192 by providing signals to the motor via the motorpower cable 196. The processor receives the signals from the pH sensor160, the chlorine sensor 170 and the temperature sensor 180 and providesthe signals as formatted output signals via the cable 116. For example,the output signals may be provided via a standard two-wire RS-232current loop via the connector 560. The processor further controls theinformation displayed on the LCD 530. The processor is responsive tosignals received from the set 522 of membrane switches on the displayand switch panel 500. For example, the processor advantageously displaysa user menu on the LCD, and a user is able to scroll through the menuusing a scroll up switch, a scroll down switch, a scroll left switch anda scroll right switch. The user is able to select menu options and entervalues using the scroll switches and a select switch. The user is ableto return to previous menu options with a menu back switch.

In an alternative embodiment wherein the control of the sensor system100 and the monitoring of the measurements are done entirely by a remotesystem via the cable 116, the LCD panel and the membrane switches may beomitted. In such an embodiment, the proximal surface of the electronicsenclosure 110 is covered with a blank panel (not shown).

FIG. 27 illustrates a simplified block diagram of the electricalcircuitry of the system 100. The set 522 of the membrane switches andthe LCD 530 are connected to the processor 600 as described above. Theprocessor includes a system controller 650, which is advantageously aprogrammable processor that communicates with other devices viaconventional input/output ports.

The system controller 650 is coupled to a motor controller 660, whichgenerates three motor phase signals φA, φB and φC and provides thesignals to the motor 192 via the motor control cable 196 in a selectedsequence to control the rotation of the motor.

The system controller 650 is coupled to a pH sensor interface 670 thatmonitors the pH sensor 160 via the cable 162. For example, the pH sensorinterface advantageously includes an analog-to-digital converter toconvert the measured voltages from the pH sensor into a digitalrepresentation of the measured voltages.

The system controller 650 is coupled to a chlorine sensor interface 680that applies voltages to the chlorine sensor 170 on an auxiliary (AUX)electrode wire in the chlorine sensor cable 172 and receives ameasurable current from the chlorine sensor via a working (WRK)electrode wire in the chlorine sensor cable. The chlorine sensorinterface shares a reference (REF) electrode wire with the pH sensorinterface 670 via the pH sensor cable 162. The operation of the chlorinesensor to apply a voltage to the auxiliary electrode while monitoringthe reference voltage and measuring the resulting current on the workingelectrode is well known to a person skilled in the art.

The system controller 650 is coupled to a temperature interface 690 thatmonitors the temperature readings on the temperature sensor 180 via thetemperature sensor wires 182.

The system controller 650 is coupled to a communications interface 700.The communications interface sends and receives signals via the cable116 and the connector 118 (FIGS. 1-5). In one embodiment, thecommunications interface is configured as a conventional RS-232interface. In another embodiment, the communications interface isconfigured as a conventional USB interface.

In the illustrated embodiment, the processor operates the chlorinesensor interface 680 synchronously with the motor controller 650 tomaintain the chlorine sensor electrodes in operational condition for anextended period. For example, as illustrated by a first timing diagramin FIG. 28, when the sensor system 100 is configured to measure seawateror to measure the water in chlorinated swimming pools, the differentialvoltage applied between the working electrode and the referenceelectrode starts at +0.25 volt at the beginning of the measurementcycle. The differential voltage remains at +0.25 volt for approximately60 seconds. During the last 2 seconds of this 60-second measurementinterval, the current is measured and averaged. The differential voltageis then reversed and the magnitude of the differential voltage isincreased so that the differential voltage is approximately −0.6 volt.This differential voltage is maintained at approximately −0.6 volt forapproximately 10 seconds to stabilize the electrodes. The measurementcycle is then repeated. During the measurement cycles, the motorcontroller operates the motor 192 to provide a consistent flow of wateracross the measurement electrodes and to move the balls 470 across thesurfaces of the electrodes to prevent the build-up of scale and othermaterials.

FIG. 29 illustrates a corresponding timing diagram for measuringdrinking water. As in the measurement cycle for seawater and chlorinatedswimming pool water, the measurement cycle for drinking water startswith the differential voltage of +0.25 volt at the beginning of themeasurement cycle. The differential voltage is held at +0.25 volt foronly approximately 5 seconds. The current is averaged over the last 2seconds of this 5-second interval. The differential voltage is thenlowered and the magnitude of the differential voltage is increased sothat the differential voltage is approximately −2.0 volts. Thedifferential voltage is maintained at approximately −2.0 volts forapproximately 1 second to stabilize the electrodes. The measurementcycle is then repeated. Several subsequent measurements may be averagedto “smooth” out noise in the measured values. This feature may beenabled or disabled from the software interface. During the measurementcycles, the motor controller 650 operates the motor 192 as describedabove to cause the balls 470 to clean the surfaces of the electrodes.

The amperometric system described above in connection with theaccompanying drawings has low maintenance requirements. The system isable to operate for extended periods unattended. The system operates forextended periods (e.g., for up to a year) without a sensor replacementor other maintenance. The chlorine sensor and the pH sensor arereplaceable individually to reduce the cost of operation. The sensoroperates in accordance with the describe method to overcome thepolarization effects of cyanuric acid (CYA). The sensor does not requireextensive recalibration, and if a sensor is removed or replaced, thesensor quickly stabilizes and reports reliable readings.

FIGS. 30-39 illustrate a further embodiment of a system 800. Inparticular. In FIGS. 30-39, elements corresponding to previouslydescribed elements are identified with corresponding element numbers.

FIG. 30 illustrates a perspective view of the sensor system 800 asviewed from the proximal end of the sensor system. FIG. 31 illustrates aperspective view of the sensor system of FIG. 30 as viewed from thedistal end of the sensor system. FIG. 32 illustrates an explodedperspective view of the sensor system of FIG. 30 viewed from theproximal end of the sensor housing. FIG. 33 illustrates an explodedperspective view of the sensor system of FIG. 30 viewed from the distalend of the sensor system. The sensor system includes an electronicshousing 810 at the proximal end and a sensor housing 820 at the distalend. As illustrated in the exploded views, a cylindrical proximalportion 822 of a cylindrical central housing portion 824 of the sensorhousing is inserted into a distal opening 830 in the electronics housingand secured therein by a threaded collar 832. As described below inconnection with FIGS. 36 and 37, a plurality of O-rings seal thecylindrical proximal portion so that water cannot enter the electronicshousing via the distal opening therein.

FIG. 34 illustrates an exploded view of the electronics housing 810 ofFIGS. 30-33 as viewed from the proximal end, and FIG. 35 illustrates anexploded view of the electronics housing as viewed from the distal end.The electronics housing comprises a proximal end portion 840 and adistal end portion 842 that are secured together by a plurality ofscrews 844. An O-ring 846 is interposed between the two portions of theelectronics housing to seal the two portions when the screws areengaged. An electronics system printed circuit board (PCB) 850 issecured to the proximal end portion by a plurality of screws 852 priorto engagement of the two portions of the housing. The proximal side ofthe electronics system PCB includes the display and switch panel 500,which includes the set 522 of membrane switches and a display window520. The display 530 (shown in FIGS. 30 and 32) on the electronics PCBis aligned with the display window as described above. The electronicssystem PCB further includes connections (not shown) to the set ofmembrane switches. The distal side of the electronics system PCBincludes a plurality of components and connectors. In particular, aconnector 860 is provided to connect with the sensor housing 820, asdescribed below.

FIG. 36 illustrates a perspective exploded view of the sensor housing820 viewed from the proximal end. FIG. 37 illustrates a perspective viewof the sensor housing viewed from the distal end. The distal end of thesensor housing supports a sensor body 870, which is similar to thesensor body 150 described above. The cylindrical central housing portion824 of the sensor housing extends from the sensor body to a proximalsensor end cap 874 which is mounted to the proximal end portion of thecylindrical central housing.

Unlike the previously described sensor body 150, the sensor body 870 inFIGS. 36 and 37 supports a modified pH/reference probe 880, which isshown in more detail in FIGS. 38 and 39. In particular, as shown in FIG.39, an electrode support portion 882 at the distal end of theph/reference probe comprises a cylindrical inner electrode 884surrounded by a concentric annular outer electrode 886. The innerelectrode is formed as a glass bulb having a flat exposed end and havingan internal electrical connection. The inner electrode and the outerelectrode are spaced apart by a selected distance (e.g., approximately0.03 inch). A conventional pelon strip 888 is positioned betweenportions of the two electrodes. The electrical connection within theinner electrode is the measurement electrode. The outer electrode is thereference electrode. An electrical connection to the inner electrodeextends to a center contact 892 of a plug 890 to provide a connection tothe measurement electrode. An electrical connection to the outerelectrode extends to an outer shell 894 of the plug. In the illustratedembodiment, the plug is a conventional RCA phono plug. The ph/referenceprobe has a hard cylindrical outer shell 896 that maintains the plug ina fixed relationship to the inner and outer electrodes.

As further illustrated in FIGS. 34 and 35, the sensor housing 820supports a sensor housing printed circuit board (PCB) 900. The sensorhousing PCB has a proximal face 902 (FIG. 37) and a distal face 904(FIG. 36). The distal face supports a first connector 910 that isconfigured as a mating jack to the plug 890 so that the plug isengageable with the jack. The distal face also supports a connector 912that receives a pair of contacts 914 on a pair of lines 916 (FIG. 36)from a chlorine sensor (not shown) that corresponds to the chlorinesensor 176 in the previously described embodiment. The distal face alsosupports a connector 920 that receives a connector 922 on a set of wires924 to an impeller motor assembly 926 that corresponds to the impellermotor assembly 190 described above. The distal face also supports aconnector 930 that receives a set of temperature sensor wires 932 (FIG.37).

The proximal face 902 of the sensor housing PCB 900 supports a connector940 that is electrically connected to the four connectors 910, 912, 920,930 on the distal face of the PCB. The connector on the proximal face ofthe printed circuit board extends through an opening 942 of the proximalend cap 874 that is secured to the proximal end of the cylindricalcentral housing 824 of the sensor housing. The end cap is secured to theproximal end of the cylindrical portion by a washer 950 and a hex nut952. The hex nut is threaded onto a rod 954 that extends from the sensorbody 870 through a bore 956 in the end cap. When the hex nut istightened, the sensor housing forms a rigid structure from the distalend to the proximal end, thus preventing stresses on the glass innerelectrode 884 of the ph/reference sensor 880. An O-ring 960 in a groove962 on the sensor body provides a watertight seal with respect to thesurrounding cylindrical central housing.

When the proximal end 822 of the cylindrical portion 824 of the sensorhousing 820 is inserted into the distal opening 830 of the electronicshousing 810, the connector 930 on the proximal surface 904 of the sensorhousing PCB 900 engages the connector 860 on the electronics system PCB850 within the electronics housing. The end cap 874 includes a flat 980that is aligned with a corresponding flat (not shown) within the distalopening of the electronics housing in order to fully insert the end capinto the distal opening. The alignment of the two flats assures that theconnector on the proximal face of the circuit board within the sensorhousing is properly aligned with the mating connector within theelectronics housing. In addition a pair of pegs 982 fit into a pair ofholes 984 in the end cap and fit into a corresponding pair of holes (notshown) in the electronics system PCB to provide additional assurance ofalignment of the two connectors. All of the electrical interconnectionsbetween the sensor body 830 and the electronics PCB are completed withone connector engagement step. After engaging the connectors, thethreaded collar 832 is engaged with the threads within the distalopening of the electronics enclosure to provide a mechanically secureand watertight connection between the sensor housing and the electronicsenclosure. An O-ring 990 in a first groove 992 on the proximal endengages the inner surface of the distal opening of the electronicsenclosure, and an O-ring 994 in a second groove 996 engages the innersurface of the threaded collar to complete the watertight seal.

As described above, the sensor system 800 is modular such that thesensor housing 820 can be easily removed from the electronics housing810 and replaced with a sensor housing having components configured fora different application,

Although described herein with respect to a sensor for a pool or a spa,it should be understood that the sensor 800 can be advantageously usedto measure chlorine levels in other applications. For example, thesensor can be used to analyze the chlorine levels in the ballast tank ofa ship so that when the ballast is pumped overboard as the ship isfilled with cargo, the chlorine levels are within an environmentallyacceptable range. As another example, the sensor disclosed herein can beused to monitor chlorine levels in drinking water.

While the description describes a sensor used for chlorine measurement,it may also be used for the measurement of bromine or TRO (totalresidual oxidant-chlorinated seawater that may contain bromine or bothchlorine and bromine). With minor changes this sensor of the instantinvention may be modified to measure peracetic acid, hydrogen peroxideand dissolved oxygen as well.

The sensor system disclosed herein may be used to implement a method forquantifying two species in water as illustrated by a flow chart 1000 inFIG. 41. In an action block 1010, the system applies a firstdifferential measurement potential between at least first and secondelectrodes of at least one sensor probe. Then, in an action block 1012,the system measures a first output signal responsive to theconcentration of a parameter of water to be measured and responsive tothe first differential measurement potential. The system then applies asecond differential measurement potential between the at least first andsecond electrodes of the at least one sensor probe in an action block1014. The second differential measurement potential is different fromthe first differential measurement potential. In an action block 1016,the system measures a second output signal responsive to theconcentration of the parameter of water to be measured and responsive tothe second differential measurement potential. In an action block 2020,the system determines a ratio of the first output signal to the secondoutput signal. In an action block 1022, the system determines which ofthe two species is present based on the ratio of the first output signalto the second output signal. In action block 1024, the system calculatesand quantifies the species determined to be present.

In an optional embodiment (illustrated by dashed lines in FIG. 41), thesystem determines whether both species are present in a decision block1030. If both species are present, then, in an action block 1032, thesystem quantifies each species according to the ratio of the firstoutput signal to the second output signal. If both species are notpresent, the system concludes the method without executing the actionblock 1032. In certain aspects of the method, the first speciescomprises free chlorine and the second species comprises chloramine; andthe ratio of the first output signal to the second output signal has avalue in a first range when the species in the water comprises freechlorine and has a value in a second range when the species in the watercomprises chloramine.

The instant invention is capable of the measurement of up 500 ppm of TROin seawater over repeated experimental runs, without suffering a loss ofsensitivity.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that all thematter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:
 1. An sensor system for measuring at least oneparameter of water, comprising: at least one sensor probe positioned influid communication with water having the parameter to be measured, theprobe comprising a plurality of electrodes, the sensor generating anoutput signal responsive to the concentration of the parameter to bemeasured; a control system electrically coupled to the sensor probe thatapplies differential voltages between at least a first electrode of theplurality of electrodes and a second electrode of the plurality ofelectrodes, the control system configured to generate a selected firstdifferential measuring voltage in a first range between −0.2 volt and+0.5 volt between the first electrode and the second electrode for afirst duration, and to generate a selected second differential voltagein a second range between 0 volt and −5 volts between the firstelectrode and the second electrode, the second differential voltageselected to be more negative than the first differential measuringvoltage, the second differential voltage applied for a second durationof at least 0.1 second following the first differential measuringvoltage, the second duration being less than the first duration.
 2. Thesensor system as defined in claim 1, wherein the first electrode and thesecond electrode each comprise platinum.
 3. The sensor system as definedin claim 2, wherein the first electrode and the second electrode areplanar electrodes deposited on a nonconductive substrate.
 4. The sensorsystem as defined in claim 2, wherein the water includes Cyanuric acid.5. The sensor system as defined in claim 1, wherein at least one of thefirst electrode and the second electrode comprises gold.
 6. The sensorsystem as defined in claim 1, wherein the water comprises seawater. 7.The sensor system as defined in claim 1, wherein the water includesgreater than 1,000 parts per million (ppm) of sodium chloride.
 8. Amethod of prevention of passivation in an amperometric sensor system,comprising: positioning at least one sensor probe in fluid communicationwith water having a parameter to be measured, the probe comprising aplurality of electrodes, the sensor generating an output signalresponsive to the concentration of the parameter to be measured; andelectrically coupling a control system to the sensor probe to applydifferential voltages between at least a first electrode of theplurality of electrodes and a second electrode of the plurality ofelectrodes, the control system configured to generate a selected firstdifferential measuring voltage in a first range between −1.0 volt and+0.5 volt between the first electrode and the second electrode for afirst duration, and to generate a selected second differential voltagein a second range between 0 volt and −5.0 volts between the firstelectrode and the second electrode, the second differential voltageselected to be more negative than the first differential measuringvoltage, the second differential voltage applied for a second durationof at least 0.1 second following the first differential measuringvoltage, the second duration less than the first duration.
 9. A methodas defined in claim 8, wherein the water includes cyanuric acid.
 10. Themethod as defined in claim 8, wherein: the water is seawater; theparameter to be measured is an oxidant in the seawater; and theplurality of electrodes includes a solid-state reference electrode andthe surface of at least one of the electrodes is coated with platinum.